1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for exposing stimulable phosphor sheets to radiation passing through an object to have a radiation image of the object stored therein, exposing the stimulable phosphor sheet to stimulating rays which cause them to emit light in proportion to the stored radiation energy, and detecting and converting the emitted light into electric signals. This invention particularly relates to a radiation image recording and read-out apparatus in which the stimulable phosphor sheets are circulated and reused for recording radiation images.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, with regard to a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load such a mobile X-ray diagnostic station with a number of stimulable phosphor sheets, and the amount of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, by storing the radiation images of the objects in the stimulable phosphor sheets, transferring the electric image signals read out from the stimulable phosphor sheets into a recording medium having a large storage capacity, such as a magnetic tape, and circulating and reusing the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examinations. This is very advantageous in practical use.
From the aforesaid viewpoint, the applicant proposed in Japanese Patent Application No. 58(1983)-66730 a built-in type radiation image recording and read-out apparatus comprising:
(i) a circulating and conveying means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path, PA1 (ii) an image recording section positioned on said circulation path for recording a radiation transmission image of an object on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object, PA1 (iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein in said image recording section, and a photoelectric read-out means for detecting light emitted from said stimulable phosphor sheet scanned with said stimulating rays to obtain an electric image signal, and PA1 (iv) an erasing section for, prior to the next image recording on said stimulable phosphor sheet for which the image read-out has been conducted in said image read-out section, exposing said stimulable phosphor sheet to erasing light or heat to release the radiation energy remaining in said stimulable phosphor sheet, PA1 (i) a circulating and conveying means for conveying at least one stimulable phosphor sheet for recording a radiation image therein along a predetermined circulation path, PA1 (ii) an image recording section positioned on said circulation path for recording a radiation image of an object in said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object, PA1 (iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein at said image recording section, and a photoelectric read-out means for detecting light emitted by said stimulable phosphor sheet scanned by said stimulating rays to obtain an electric image signal, PA1 (iv) an erasing section for, prior to the next image recording in said stimulable phosphor sheet for which the image read-out has been conducted at said image read-out section, exposing said stimulable phosphor sheet to erasing light or heat to release the radiation energy remaining in said stimulable phosphor sheet, PA1 (v) a time measurement means for measuring the time elapsed after radiation energy remaining in each stimulable phosphor sheet is released, PA1 (vi) an image recording detection means for detecting the image recording on each stimulable phosphor sheet, and PA1 (vii) a control means for receiving outputs of said time measurement means and said image recording detection means and activating said circulating and conveying means to send said stimulable phosphor sheet to said erasing section when said elapsed time of said stimulable phosphor sheet reaches a predetermined length prior to image recording on said stimulable phosphor sheet.
whereby said stimulable phosphor sheet is circulated through said image recording section, said image read-out section and said erasing section and reused for radiation image recording.
As described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 or U.S. Pat. No. 4,400,619, the radiation energy remaining in the stimulable phosphor sheet is released when the sheet is exposed to light or heat. However, after the residual radiation image is erased in this manner, a new residual image is generated as time elapses by radiation energy which has not been released completely. The stimulable phosphor sheet is also affected by environmental radiations such as cosmic rays and X-rays emitted by other X-ray sources and stores the energy thereof. These types of radiation energy (referred to as fog) undesirably stored in the stimulable phosphor sheet also cause noise to appear in the visible image reproduced from a reused stimulable phosphor sheet and, therefore, must be erased before reusing the stimulable phosphor sheet.
Therefore, in the aforesaid radiation image recording and read-out apparatus of Japanese Patent Application No. 58(1983)-66730, when a long time, e.g. several hours, has elapsed between erasing of a residual image in the stimulable phosphor sheet conducted at the erasing section after image read-out and the next radiation image recording on the sheet, it is necessary to circulate the sheet and pass the sheet through the erasing section and to conduct secondary erasing prior to the next image recording. As described, for example, in Japanese Unexamined Patent Publication No. 57(1982)-116300, the erasing time in the secondary erasing may be shorter than the erasing time in primary erasing conducted immediately after image read-out. However, it takes some time to circulate the sheet. In general, since it is not always possible to know how many hours have elapsed after the sheet which is to be used for image recording was subjected to erasing of the residual image in the radiation image recording and read-out apparatus, the secondary erasing has heretofore been conducted nearly always by circulating the sheet before the apparatus is used. This operation is troublesome and requires a long time.